The United States Department of Defense has mandated that munitions be designed to withstand unplanned stimuli and improve survivability throughout its life cycle. Specifically cook-off or temperatures higher than operating temperatures are one of these unplanned stimuli and the proposed innovation for the lifting plug addresses this issue. The U.S. Army and U.S. Marine Corps field artillery units are equipped with the M109A6 Self Propelled Howitzer, M198, and/or M777A2 Joint Lightweight Towed Howitzers that use the M795 High Explosive (HE) projectile. This M795 projectile is a typical round that is packaged and transported with a lifting plug and before use, a fuze replaces the lifting plug to make the round ready for its mission. Additionally, there are other projectiles that use the lifting plug as a transport and drop indication means.
These munitions have a threaded fuze cavity (not shown) into which can be inserted a lifting plug shown generally at 37 in FIGS. 1A and 1B. Lifting plug 37 comprises lifting ring 39 connected to neck 41 and threaded round portion 43.
The lifting plug is used as a shipping means to not only lift the munitions but to also protect the munitions from the environment until the fuze is attached. In protecting the munitions until the fuze is attached, the lifting plug seals the munition, and, under high temperatures, the sealing of the munition may cause a high-order detonation under high temperatures.
In general, munitions have operating temperatures between −60° F. to 160° F. In case of a fire, the temperature of the munition raises beyond a safe operating temperature of, for example, 160° F., and the energetics inside the munition phase change from solid to liquid causing an internal “hoop pressure” that causes the munition to explode in a high order reaction. This reaction is undesired and if the internal hoop pressure could be relieved, the energetics would not undergo a high order reaction or detonation but instead a combustion or burning, which is deemed safer for firefighting and overall physical damage/destruction.
The severity of a high order energetic reaction is the desired effect or function for explosives confined in thick steel walled projectiles. This combination creates the lethality and fragmentation requirements for munitions under standard operational scenarios but in non-operational scenarios, such as when munitions are exposed to elevated temperatures, the energetics can self-detonate and create undesired explosions which can cause loss of life, material damage and destruction of facilities and transport vehicles. In these elevated temperature scenario's, venting is critical to limit the warhead reaction reducing the high order reaction into a combustion or burning reaction. Accordingly, projectile modifications are desired to incorporate venting means for the energetics under these non-operating temperature exposures.
Several different lifting plug designs have been developed and tested over the years. Venting and drop indicator features have always been a consideration but have not been successful in achieving both characteristics. Prior art lifting plugs shown in FIGS. 1A and 1B are designed to be able to;                a) Support the weight of the round or pallet during transportation.        b) Deform or break if the round has been dropped with significant energy imparted to potentially cause structural damage to the ogive; thus giving a visual drop indicator.        c) Seal the round fuze cavity during the packaging transportation of the round until the fuze is assembled.        
The drop indicator capability is illustrated in FIGS. 2A and 2B. The drop indicator capability is an essential feature of a lifting plug, used to identify rounds that have the potential of detonating within the cannon tube due to cracks in the energetic material.
The neck 41 can be formed of a frangible or weakened material which is designed to either rupture the neck 41 as shown in FIG. 2A or deform the neck 41 as shown in FIG. 2B when the munition is subjected to rough handling which might damage the munition itself and create an unsafe and/or unstable munition.
Some prior art lifting plug designs incorporate wax material that would soften at temperatures below the explosive reaction temperature to reduce confinement and prevent transition from deflagration to detonation, such as shown in FIG. 3. However, incorporation of the wax material resulted in degradation in the structural integrity of the lifting plug and its ability to accurately indicate excessive force impact to the ogive. This premature lifting plug failure results in a lower projectile availability to the users if projectiles were to experience rough handling in transportation. Wax material venting designs frequently have small vent holes, limiting the ability of the plug to vent energetic material.
In one approach, lifting plug 37 was modified to include a venting method using a wax material (FIG. 3). This lifting plug with a wax material was found to be ineffective since it interfered with the visual indicator provided by weakened neck 41 as illustrated in FIGS. 1A and 1B.
Similarly, designs utilizing threaded rings, such as the lifting plug disclosed in U.S. Pat. No. 8,596,291 limit the size of the vent opening, limiting the ability of the plug to vent energetic material.
Therefore, there remains a need for a lifting plug, capable of identifying rounds that have the potential of detonating within the weapon or cannon tube due to cracks in the energetic material, and that can vent the energetic material during cook-off or temperatures higher than normal operating temperatures.